Gas turbine engines, such as those which power aircraft and industrial equipment, employ a compressor to compress air that is drawn into the engine and a turbine to capture energy associated with the combustion of a fuel-air mixture. The compressor and turbine employ rotors that include multiple airfoil blades mounted on, or formed integrally with, rims of a plurality of disks mounted on a shaft. Typically, such shafts are rotatably supported on bearings and are lubricated with a lubricant. For example, oil may be disposed within an interior of a bearing compartment.
Referring to FIG. 2, it is known to provide a system 200 that includes a bearing compartment 204 with mechanical seals, such as non-contacting face seals, to reduce (e.g., minimize) the escape of lubricating fluid from forward and aft ends of the bearing compartment. The air outside of these ends is typically at a higher pressure than the pressure of an air-oil mixture inside the bearing compartment 204. Face seals typically employ a stationary carbon seal 210 and a rotatable seal plate 216 mounted on a rotor shaft 222. The carbon seal 210 is usually provided with a smooth, continuous (uninterrupted) sealing surface which is disposed in a face-to-face, opposed relationship to a sealing surface of the seal plate 216. A spacer 228 maintains the seal plate 216 in axial alignment. Like the seal plate 216, the shaft 222 and the spacer 228 are configured to rotate.
The sealing surface of the seal plate 216 is often equipped with hydrodynamic (so-called “lift-off”) features 234, such as with a pattern of spiral grooves. A source 240 of fluid (e.g., air), which is taken from the compressor or a core primary/combustion flowpath, enters the grooves 234 at the entrainment location 246. The fluid then exits the grooves 234 and consumes at least a portion of a space between the carbon seal 210 and the seal plate 216 from outside the bearing compartment 204. The fluid is pumped within the spiral grooves 234, raising the pressure thereof such that the elevated pressure of the fluid within the grooves 234 forms a fluid barrier between the carbon seal 210 and the seal plate 216 thereby restricting the leakage of the air-oil mixture from inside the bearing compartment 204 into the space between the carbon seal 210 and the seal plate 216. The pumping characteristics of the grooves 234 to provide the elevated pressure fluid seal between the carbon seal 210 and the seal plate 216 is a function of the geometry of the grooves 234, the rotational speed of the seal plate 216 and the characteristics of the fluid supplied to the grooves 234 at the entrainment location 246.
Since gas turbine engines operate at a wide range of rotational speeds, the ability of the grooves 234 to provide the pressurization of sealing fluid between the carbon seal 210 and the seal plate 216 over a wide range of rotational shaft 222 speeds is imperative. However, when the pressure of the source fluid 240 is relatively low (such as for example at high altitude, sub-ambient pressure conditions) the low density of the fluid 240 compromises the ability of the grooves 234 to generate sufficient pressure.